DE2831212A1 - Blown film prodn. control - e.g. in revolving tube process, controls air ring supply responsive to pyrometer signal below frost line - Google Patents

Abstract

Extruded and blown plastic film process is monitored and controlled according to film temp. at a point between the extrusion die and the frost line. A signal form a temp. sensor is compared with a datum. The error signal is used to control cooling of the film. Used e.g. for a revolving tube type process. The control compensates for cyclic variations which can arise e.g. variation of melt index and melt temp. conditions in cooling system, gradual clogging of filter. A useful increase in prodn. rate is obt. without adverse effect on quality, or better quality without increased rate.

Description

This invention relates to a method of manufacture

a blown film made of a plastic material. In this procedure the film is made by extrusion of a tube by heating softened plastic, stretching or pulling this hose over an enclosed one Bubble of air or another gas and by cooling the tube at the same time, such as external or internal cooling devices.

The invention is particularly directed to a method in which a control area is defined, a control temperature is determined and in which Control range is maintained by variable coolant, creating a Film of improved quality and / or higher production speeds achieved will. Alternatively, the invention can also be characterized such that it is a controlled position of the frost line defined in the hose-like gas film, this position is monitored and variable cooling devices are available and corrects deviations from this frost line for better results.

The properties and the quality of the finished blown film can in particular due to typical fluctuations in the operating conditions and irregularities and deviations of the extruded plastic can be influenced.

For example, the extrusion temperature tends to be an inherent exhibit cyclic character. Cyclic conditions also occur under usual conditions Means for cooling the film, regardless of whether this is a cooling source or Ambient air is used. In addition, film extruders often require a filter system, which is gradually clogged by impurities, which is also a variable Effect arises, in particular with regard to the change in the melt index ', at Passage of the plastic through the filter. In addition, the plastic itself can not completely uniform in quality and may be different Have melt indices and / or melting temperatures.

Such fluctuations in the operating conditions and in the starting material can result in poor quality movies, such as the appearance of Wrinkles and poor film thickness uniformity. The film thickness can be adjusted in one to a certain extent are controlled by measuring devices that are available to the operating personnel give a note and a warning if the film thickness or the film profile has changed changes so that appropriate corrections can be made. However, if the When operating conditions deteriorate, wrinkles appear in the film. So far could however, no reliable control devices that provide a prediction are developed allow the deteriorating operating conditions in this direction, so that the operating staff would receive sufficient warning to make appropriate corrections to undertake.

Even with close monitoring by trained operating personnel So far it has been difficult to maintain the film quality above certain production speeds to control.

Even if the production speed is so low -lag, that it could actually have been checked by professional staff, occurred in the film quality due to the inadequate control devices and due to cyclic and / or fluctuating operating conditions still malfunctions in the film quality.

The object of the invention is therefore to provide a method to ask, with the help of which you can get a film of better quality and superior Can produce properties.

This object is made finer and more precise by the invention Control during film production solved.

The invention therefore relates to a method for production a blown film by continuously extruding a tube from a film-forming, plastic softened by heating, stretching or pulling the tubular Film over a trapped bubble of air or some other gas, this being this Method is characterized by monitoring the temperature of the tubular Film in a control area that extends between the frost line in the tubular Film and the extrusion head extends, whereby this control area extends enough away and at a distance from the frost line so that it passes through the less Crystallization of the material softened by heating is affected; Determine a control temperature for the control area; Compare the monitored temperature with the set temperature by controlling the temperature of the film around its circumference in the control area and above the extruder head as a function from the monitored temperature to a substantially constant temperature in the Maintain control area of the blown film.

The method of the invention enables the manufacture of a blown film of higher quality and the operation of such an extrusion line at significantly higher speed while maintaining an acceptable level of film quality.

In the invention, a selected control area of the extruded Films or, alternatively, the position of the film frost line is monitored. It becomes a control temperature or determines a position of the control frost line, at which a film of good, preferentially achieved optimal quality over all comparable options will.

A variable cooling source is regulated depending on signals, which it receives from the monitoring device to determine the control temperature or the To maintain control position essentially as a constant operating condition. This control system leads to a more precise automatic control of the film properties.

It is especially beneficial to get a quick fix for a condition induce loss of flatness of the film, d. H. for wrinkling, would lead. Such warning and control systems have so far not been more satisfactory Shape available.

The method according to the invention is hereinafter referred to explained in more detail on the schematic drawing. A thermoplastic plastic is introduced into a heated extruder 10 via a hopper 12, and is after softening by heating via the connecting line 16 to the extruder head 14 supplied. The plastic leaves the extruder head in the form of a continuous Film tube 18. The tube is made by an entrapped bubble of air or another gas stretched or drawn, the amount of gas being maintained or is supplemented by the regulated pressure line 20.

With the help of a folding frame 22 and the rollers forming a slot 24 and 26 the film becomes final in an area remote from the extruder head folded. Typically, in the device used here, they are also not shown Drive rollers are present, which are arranged after the nip rollers 24 and 26 and which is the driving force for conveying the hose from the extrusion tool to the Provide. The speed of the drive rollers is set so that the film tube is stretched or pulled in the longitudinal direction and this speed factor together with other control factors determines the size of the circumference of the hose, d. H. whether the circumference increases, decreases or essentially corresponds to the extruded one Size remains. The stretch takes place essentially between the extruder head and the film frost line 28. Above the frost line the film has solidified or is in a semi-solidified state.

A temperature sensing device 30 is set so that it the Read the temperature of the film continuously and monitored in one Control area, which is below the frost line 28 and above the extruder head 14 is located. The film area immediately adjacent to the frost line is not like this well suited as a control area.

This is likely due to the effect of film crystallization in the vicinity of the frost line, as here the tendency for a permanent one Temperature value exists or a value that does not reflect the changing conditions shows in a satisfactory manner so that an automatic correction can take place. However, there is a good control area for prediction and control in an area that is sufficiently downward from the frost line that it is through does not affect the crystallization effects at and near the frost line will.

The signal generated by the temperature sensing device 30 is Via an electrical line 34 to a control device 32 and further to a temperature reading device or a temperature recorder 36 via the additional line 38 is supplied. The temperature device 36 converts the signal into a readable quantity, whereby the Temperature in the control area can be determined numerically at each operating time can.

The output from the control device 32 is designed so that he via a pneumatic or electrical line 42 an air or electrical Motor or an actuator 40 for a shut-off element can operate.

The adjusting device for the closing element is through a suitable Connecting device 44 is connected to a throttle valve 46. The throttle valve is in line 48, the cooling air or another gas from a blower or compressor unit 50 approaches, rotatably mounted. The air or the other gas is the cooling ring 52, the one around the extreme lower end of the hose just above the extruder head 14 is arranged, supplied.

The system depends on the selection or determination of the control temperature for the control area. This is expediently determined empirically by by trying out a number of operating conditions, one can find those in which a film of optimum quality is obtained. After identifying this Conditions will be the optimal temperature in the control area as the control temperature given. The one read in the temperature device and, if necessary, written down Temperature is not essential for the functioning of this control device, however, it enables the operating personnel to track the temperature. the The control device is set in such a way that it is continuously monitored by the sensing device 30 compares the signal obtained with the control temperature or an equivalent predetermined control signal. When the signal from the sensing device indicates that the temperature rises in the monitored area, this is determined by the control device 32 perceived, which changes the adjusting device 40 so that the throttle valve a greater supply of cooling air to the ring 52 is allowed to keep the temperature in the control area to lower until the submitted control temperature is reached. If alternatively the Sensing device reports a drop in temperature in the control area, reacts the control device so that the throttle valve allows the supply of cooling air reduced to the cooling ring 52. The so-called zero position, that is, the position which the valve occupies when the control area is at the submitted temperature is of course a point between the extremely open and the extreme closed position of the throttle valve.

The invention can also be carried out in such a way that the height or the position of the frost line is used as a control indicator. Thereby leaves the monitoring device changes in such a way that the height of the frost line is visible or is read in some other way and that appropriate signals are triggered, which show a deviation from the target line. The control position of the frost line can also be determined empirically by a range of operating conditions used to determine the frost line at which a film of optimal Quality arises. The signal emitted by the monitoring device is also in this case forwarded to the control device and with the submitted control signal compared. In this case too, the control device then makes a correction made to increase the supply of cooling air through line 48 or to reduce so that the submitted position of the frost line is maintained.

This control or control procedure can generally be found in the Manufacture of blown films from film-forming plastics produce. Typical examples of such film-forming materials are thermoplastic film-forming materials Plastics such as polyethylene, copolymers of ethylene with other monomers such as Propylene, acrylic acid, ethyl acrylate, polypropylene and propylene copolymers, polyester, Polystyrene, styrene copolymers, polyvinyl chloride, vinyl chloride copolymers, vinylidene chloride copolymers and polyamides.

Known sensing devices can be used as the monitoring device use for temperature measurements or optical measurements, such as optical pyrometers or radiation thermometers, thermocouples or thermistors of the spring-probe type. If the control is based on monitoring the position of the frost line, an opacimeter can be used to monitor the position of the frost line and a signal for regulating the cooling air from the corresponding source give.

The control device 32 compares the input signal from the sensing device or monitoring device 30 with a control signal and gives an output signal, any deviation between the input signal and the control signal, if any is proportional.

The adjusting device 40 for the valve can depending on the Input signal, the available space and the type of valve electrical or be pneumatically driven. The throttle valve 46 may be through others adjustable valve types be replaced or by other institutions, those for regulating the flow or the temperature of the cooling gas for the cooling ring 52 are suitable. A blower or a fan can do the job Provide cooling gas. The cooling ring 52 is arranged in an area by most effectively influencing the temperature of the film in the monitored area can. The air ring is preferably located in the vicinity of the extruder head 14, as in the drawing.

Instead of or in addition to the cooling ring 52, other cooling devices can be used be used, e.g. B. arranged inside the hose cooling devices.

The method according to the invention is so simple and flexible that that there is no difficulty for the various methods of making Blown film can be customized. You can z. B. a rotatable extruder head or a rotatable pick-up device or similar devices for continuous Use rotation of tube 18 as used in making blown film known from certain plastics and for the manufacture of certain end products is. The method according to the invention can be used in such a manner of operation use essentially without modification. Under certain conditions it can however, it may be desirable to have multiple monitoring areas when rotating Provide film tube, and / or use an integrator device to obtain an average value of the temperature in the monitored areas and / or to provide a change in cooling only at specific intervals, e.g. B. after each complete rotation of the film. Naturally, the skilled person will be the monitoring procedure after the ore in special features adapt to the individual blowing process for film production.

In addition to the advantages already mentioned, the process stands out according to the invention still characterized by the fact that more uniform properties of the film quality also obtained with regard to the strength and the shrinkage behavior of the film will.

Example 1 This example shows the preparation of a polyethylene film in a blow molding process with a rotating film tube (revolving tube type) with a diameter of 50.8 cm. To sense the temperature of the film, a commercially available non-contact optical pyrometer or radiation thermometer (Ircon Modlin "Instrument Series 3400") was used. The monitoring area is at least 3 inches below the frost line and preferably 23 inches below the frost line and at least 15 cm above the extruder head. The submitted control temperature is 115.60C. A commercially available device (IrconÖR proportional controller, Instrument Series 3400). This device picks up continuously the electrical outputs of the optical pyrometer and converts them proportionally into a pneumatic output, which is an air piston motor with an associated Throttle valve drives. This part of the facility is also commercially available (~ Valtek Vector One Butterfly Valve "). A blowing device is used for the cooling gas used with a capacity of 42.48 m3 / min (1500 ft3 / min). This blower will operated at full capacity and the degree of cooling is regulated by the setting of the throttle valve.

A comparison experiment with the invention is compared in Table I. The "maximum rate" is the maximum achievable film production that but because of the lack of quality it is not considered for commercial use comes. The maximum output that provides a good film in terms of the flatness of the film and its thickness sufficient for commercial claims.

Table I comparison invention maximum rate kg / h 306 306 maximum good production kg / h 249 294 Example 2 The control method according to the invention is used for the production of a blown film made of polyethylene with a diameter 76.2 cm. The blown film is produced with a rotatable Hose. The control method and apparatus are essentially the same as in example 1. The same also applies to the control temperature and the control area to. In this case too, a comparative test is made for a production from Blown film carried out without a control device. By using the inventive Control device is also in this example a significant increase in maximum good production achieved.

The results are summarized in Table II.

Table II comparison invention maximum rate kg / h 454 454 maximum good production kg / h 363 431 Blank page

Claims (2)

METHOD FOR PRODUCING A BLOW FILM Patent claims: Method for the production of a blown film by continuously extruding a hose from a film-forming plastic softened by heating, stretching or pulling the tubular film over an enclosed air or other gas bubble, characterized by monitoring the temperature of the tubular film in one Control area that is between the frost line in the tubular film and extends the extrusion head, this control area being sufficiently removed and located at a distance from the frost line so that it is less affected by the crystallization the material softened by heating is affected; Establishing a control temperature for the control area; Compare the monitored temperature with the set one Temperature by controlling the temperature of the film around its perimeter in the control area and above the extruder head, depending on the monitored temperature, by one substantially constant temperature in the control area of the blown film upright obtain. 2. The method according to claim 1, characterized in that the speed of the flow of cooling gas or air passing through a placed in the control area Ring flows over the extrusion head, is varied.